The present invention concerns a system and method for detecting misfires of cylinders of an internal combustion engine. The invention contemplates the detection of misfires or lack of combustion of individual faulty cylinders of an engine, and particularly for engines having a microprocessor based electronic control module, or ECM.
The Clean Air Act of 1975 sought to control exhaust emissions from internal combustion engines for light duty motor vehicles. In response to that Act, most automotive manufacturers have employed catalytic converters to control the emission of carbon monoxide, hydrocarbons and other noxious gases. Recently, regulatory agencies have proposed that passenger, light-duty and medium-duty vehicles incorporate some form of indicator to indicate a malfunction of an emission-related component that interfaces with the on-board engine control computer or microprocessor.
Misfire of engine cylinders can damage the catalyst of a catalytic converter. Some regulations for spark-ignition engines require identification of a misfiring cylinder as well as a percent misfire over a predetermined number of engine cycles. Other regulations have specifically targeted diesel engines. For example, the California Air Resources Board has initiated an On Board Diagnostics program, OBD II, that requires monitoring of each cylinder at least once per driving cycle. The OBD II regulations require that the diagnostic system be able to identify multiple faulty cylinders and provide a driver-observable indication of the fault condition.
Many techniques and systems have been proposed to detect and identify faulty cylinders. Some of these techniques are summarized in SAE Paper No. 960039, entitled "An Overview of Misfiring Cylinder Engine Diagnostic Techniques Based on Crankshaft Angular Velocity Measurement". The approaches summarized in this technical overview generally concentrate on crankshaft speed and angular velocity or torque variations.
Other approaches are briefly discussed in U.S. Pat. No. 5,529,041 to Andrews, owned by the assignee of the present invention. The systems summarized in the '041 Patent are characterized as "passive" monitors, meaning that they monitor each cylinder's contribution to engine speed under normal fueling conditions and register a misfire upon detection of a characteristic deceleration. As noted in the background of the '041 Patent, which text is incorporated herein by reference, passive systems are susceptible to certain errors, such as false positive failures, and often incapable of detecting "weak" misfire conditions such as may occur at low idle.
In response to the noted deficiencies of the prior passive systems, the inventor in the '041 Patent proposed an "active" misfire detection strategy that was inherently more robust than the prior systems and less likely to incorrectly detect engine misfires. The details of this prior inventive system can be discerned from the specification of the '041 Patent, which is also incorporated herein by reference. In brief, the '041 Patent describes an active monitoring system that senses engine rotational speed at predetermined crank angles corresponding to specific cylinders. The active aspect of the invention of the '041 Patent involves providing a quantity of fuel to the cylinder being tested that exceeds the fuel demanded by that cylinder. If the engine speed at the specific crank angle does not increase in accordance with the excess quantity of fuel, then the subject cylinder is identified as misfiring.
The system and method disclosed in the '041 Patent solves many of the problems associated with the prior passive approaches. However, this active approach has its own drawbacks. For instance, since this active detection system requires overfueling a subject cylinder, the misfire detection test is designed to be conducted only once per driving cycle or vehicle trip. Moreover, since the active approach involves overfueling, some deliberate intervention is required to initiate the test sequence.
Another difficulty with the prior misfire detection systems resides in the manner in which the occurrence of a misfire is detected. For example, some systems rely upon crankshaft angular velocity, while others utilize engine speed fluctuations. These approaches can complicate the misfire detection algorithms that must be implemented by the ECM.
Moreover, none of these prior systems adequately address the variations in proper cylinder firing vs. misfiring that occur due to changes in the overall engine operating condition. In addition, the need remains for a misfire detection system and method that can provide a quantitative evaluation of the nature or degree of the misfire condition of the affected cylinder(s).